The present invention relates to the field of magnetic parts of the type used in a variety of industrial devices and other applications. More specifically, the invention relates to the field of magnetic parts exhibiting high performance characteristics in relatively high temperature environments.
The number and type of applications for permanent, molded magnets have increased rapidly in recent years, particularly in such areas as computer peripherals, defense, and telecommunications. Recently, there has especially been a need for magnets that perform well in high temperature environments. For example, for D.C. motors, particularly those in satellite systems and small military applications, temperature stability during operation is vital.
In addition to the need for magnetic parts that exhibit favorable characteristics in high temperature conditions, it is also often desirable that magnetic parts be amenable to injection molding. Injection molded magnets can be formed cost effectively and can be formed to precise shapes and dimensions customized to particular applications.
Thus, there is a need for a magnetic compound that is both amenable to injection molding and that exhibits favorable characteristics in high temperature conditions. To address this need, various bonded magnetsxe2x80x94i.e., magnets which are manufactured by mixing a plastic or rubber like material with magnetic powder and then transferring the mixture in to a moldxe2x80x94have been developed using a high temperature magnetic material, such as samarium cobalt. The plastic or rubber material in these compounds provides such compounds with the ability to be shaped and molded, while the powder provides magnetic properties that vary little in high temperature environments.
However, in order for such bonded magnet compounds to perform in high temperature applications, not only the powdered magnetic material, but also the plastic or rubbery material must exhibit favorable characteristics in high temperature environments. To address this issue, polyphenylene sulfide (PPS) (which has a low coefficient of linear thermal expansion and excellent physical strength at high temperatures) has been used as the polymer constituent in bonded magnets.
Unfortunately, during injection molding of such magnets, the PPS is necessarily melted, and during this melt phase, samarium cobalt appears to catalyze a degradation (i.e., a viscosity reduction) of the PPS. Thus, the PPS, and therefore the entire compound, is rendered degraded and more difficult to mold (thus diminishing the benefit gained from adding the polymer constituent, i.e., the PPS, in the first instance).
To address this problem, other polymers have been used with samarium cobalt (or other high temperature magnetic materials) to form various magnetic compounds. As is the case in general, these polymers render the compounds amenable to injection molding and, further, the molding properties of these polymers are not degraded during manufacture. However, unlike PPS, such polymers do not exhibit favorable characteristics in high temperature environments.
Thus what is needed is a compound for magnetic parts exhibiting favorable characteristics in high temperature environments, that is amenable to injection molding and at the same time is not degraded during manufacture (melt phase) of the magnetic compound.
The present invention addresses this need. In particular, the present invention provides a high temperature bonded magnetic compound that can also be molded into complex shapes and that does not degrade when the polymer constituent is in its melt phase. To achieve these properties, the present invention exploits the unique performance characteristics of samarium cobalt magnetic powder and PPS polymer, each of which exhibit favorable characteristics at high temperature, and combines these substances in a compound that uses a relatively impermeable, inorganic coating to prevent the degradation of PPS in melt phase.
More specifically, the magnetic compound of the present invention comprises: (1) 25% to 50% by volume of polyphenylene sulfide (PPS) and/or possibly other polymers exhibiting favorable high temperature characteristics; (2) 50% to 70% by volume of coated, powdered magnetic material exhibiting high performance characteristics in high temperature environments, such as samarium cobalt (the coating comprising 1 to 30% kaolin by weight of the coating and 70 to 99% potassium silicate by weight of the coating); and (3) 0% to 5% by volume of an internal lubricant.
The PPS polymer component of the compound renders the compound amenable to injection molding to precise tolerances and also has a low coefficient of linear thermal expansion as well as excellent physical strength at temperature. The samarium cobalt constituent of the compound provides relatively high magnetic properties and thermal stability. At the same time, the potassium silicate/kaolin coating separates the PPS from the samarium cobalt during manufacture, thus preventing degradation during manufacture.
In accordance with another aspect of the present invention, the above described compound can be manufactured by: (1) blending high temperature magnetic materials, such as samarium cobalt; (2) coating the magnetic powder with kaolin/potassium silicate; (3) compounding the coated samarium cobalt with PPS and/or similar polymers and an internal lubricant; (4) granulating the compound; (5) injection molding the granulated compound; and (6) magnetizing the resulting molded pat. The result is a high temperature, bonded magnet that is injection molded to detailed specifications, exhibits strong magnetic force and is resistant to degradation during manufacture.